Saturday, 31 December 2011

Stability Augmentation System and Pitot Tubes

Stability Augmentation System and Pitot Tubes

Both Labradors and Voyageurs use a stability augmentation system (SAS) to keep the aircraft stable in flight and in the case of 113s to keep the aft rotors from trying to catch up to the front rotors.  While the helicopter could be flown with the SAS off, it was much more challenging and certainly not as smooth a flight.  If the SAS kicked off while in the hover or unexpectedly, it made things potentially dangerous.  The two-channel SAS system utilized SAS (static) ports on both the nose and fuselage of both variants.
The rather large red circles on the nose encircle the Labrador's nose mounted static ports. The circles would eventually become smaller.
Front view of Voyageur nose with covered static ports.

Left side view of uncovered Voyageur static port
Where they differed was in the number and location of the static ports.  Both types had four ports on the nose, two on the left and two on the right. As previously mentioned, the location of those on the Voyageur differed slightly because of the extra nose glass.  The Labrador also had three static ports on either side of the fuselage, the Voyageur just two. I do not know if the third static port meant the Labrador at one time had a back up channel or if it was tied to another function?
Note the Labrador's cluster of three static ports aft of the front door. There were three others in roughly the same position on the left side of the fuselage.

The pitot static system also utilized a port to provide among other things, airspeed. When the Labradors were purchased, they were equipped with two pitot static tubes on the right side and one on the left side of the fuselage.  They were located above and aft of the cockpit doors.  Voyageurs had only the set of two pitot tubes on the right side fuselage.  At some undetermined point in time, for me anyway, the left side single pitot tube was removed from all Labradors.  At the same time, the number of SAS ports on both sides of the fuselage was reduced from three to two. On the right side of the fuselage, the SAS static ports were just aft of the lower Dutch Door and on the left just aft of the bottom of the emergency door.  Photographic evidence reveals that aircraft 304 was the last to shed the left side pitot tube but only after it had been through SARCUP. Unfortunately, I am not able to figure out how to put the picture of 304 after SARCUP into the this time anyway.!

Labrador 302, in pre-SARCUP yellow livery, with the right side static ports clearly visible in this spoof photo.  The cable cutter cartridges were for a time unreliable rendering the hoists temporarily inoperable, so a mock backup system was rigged.

Voyageur 313 in variegated camo, Note the two static ports on the right side of the fuselage aft of the front door.

Note the two static ports on the left side of Voyageur 407.

Right side pitot tube.

Note Labrador left side pitot tube just below the forward pylon door and above the cockpit door.

The left side pitot tube is barely visible on Labrador 401.

Pre-SARCUP Labrador 301 still retains the left side pitot tube.

Labrador 301, after SARCUP no longer sporting the left side pitot tube.
No left side pitot tube here.

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